Production of colourless, transparent or opaque coloured plexiglas blocks from a prepolymer that is devoid of residual initiators by thermal polymerisation

ABSTRACT

The invention relates to a method for producing polymethyl methacrylate (PMMA) blocks with thicknesses greater than or equal to 200 mm.

FIELD OF THE ART

The invention relates to a novel casting polymerization process withwhich it is possible to produce polymethacrylate blocks with a thicknessof 200 mm and thicker.

STATE OF THE ART

U.S. Pat. No. 1,942,531 discloses a process for polymerizing vinylchloride or styrene in a tubular reactor. The polymerization is carriedout in the presence of approx. 1% terpene-like substances.

DE 0 632 951 (Dr. Otto Röhm) discloses a process for polymerizingacrylic esters in the presence of compounds from the terpene series (upto approx. 1%) to give blister-free products.

It is possible with the process to produce nonsplinter glass bypolymerization of a monomer mixture between glass sheets.

Munzer et al. (Angewandte Makromolekulare Chemie 11 (1970) p. 27-40)describe the influence of carbocyclic six-membered ring compounds withtwo nonconjugated double bonds on the suppression of the Trommsdorffeffect.

Terpinolene copolymerizes with MMA and thus leads to the reduction inthe Trommsdorff effect.

DE 29 41 959 discloses a process for the free-radical polymerization ofmethyl methacrylate by first preparing a prepolymer and then destroyingthe initiator in the prepolymer with the aid of a polymerizationinhibitor, for example thioether. With the aid of the syrup, sheets upto thickness 20 mm are produced.

Sheets of PLEXIGLAS® are known and are sold commercially by Röhm GmbH &Co. KG. The thickness of the sheets is between 1 mm and 20 mm.

When thicker sheets are required for special applications, for exampleaquarium construction, these thicker sheets have to be produced byadhesive-bonding of thinner sheets. This constitutes a notinconsiderable level of cost and inconvenience. Owing to the thermalproblems in the polymerization, thicker sheets can be produced only withgreat difficulty. The heat removal entails long polymerization times.

However, even the production of sheets from PLEXIGLAS® with thicknessescustomary to date is afflicted with disadvantages.

The thermal polymerization in a combination of water bath and airpolymerization cabinet is complicated.

The necessary reaction time is extremely long. In the case of 100mm-thick blocks, 30 days at 27° C. are required.

OBJECT

It is thus an object of the invention to develop a polymerizationprocess and to find formulations which allow the production of blocksfrom PLEXIGLAS® with a thickness between 30 and approx. 500 mm by aneconomically viable process. The length and width of the blocks isapprox. 6000 mm (length) and approx. 3000 mm (width).

Solution

It has now been found that use of a residual initiator-free syrup ofpartly polymerized methyl methacrylate of the following composition

-   a) from 10 to 40% by mass of methyl methacrylate-   b) from 0.0 to 1.0% by mass of a crosslinker-   c) from 0.001 to 0.1% by mass of further compound copolymerizable    with the compounds a)-b) (carbocyclic compound)-   d) the customary release agents in from 0.01 to 1% by mass-   e) 90-60% by mass of a syrup-   f) peroxydicarbonates as an initiator for syrup preparation    allows, in the thermal polymerization process, the production of    thick blocks (a thick block is a block with thickness more than    29 mm) in hitherto unknown qualities, yields and polymerization    times.

Useful crosslinkers include the following compounds: glycoldimethacrylate, allyl methacrylate, TAC (triallyl cyanurate).

Useful copolymerizable compounds include terpenes and dimericα-methylstyrene.

The syrup used has to be initiator-free. Initiator-free is understood tomean that the initiators used to prepare the syrup, on attainment of thedesired degree of polymerization, are decomposed thermally (>90° C.) sothat they are no longer capable of initiating polymerization.

The initiators used are preferably peroxydicarbonates, preferablybis(4-tert-butylcyclohexyl)peroxydicarbonate.

The prepolymerized syrup consists of an MMA polymer dissolved in themonomer and its viscosity is adjusted such that it can be handled in thecustomary plants, for example to from 1200 mPa·s to 50 mPa·s.

The inventive mixture also comprises polymerization-regulatingcarbocyclic compounds having at least one double bond, for exampleterpenes, e.g. γ-terpinene, or dimeric styrene compounds such as dimericα-methyl-styrene.

The amount of carbocyclic compounds having at least one double bond isbetween 50 ppm and 300 ppm based on the total amount of thepolymerization batch, preferably between 80 ppm and 250 ppm and mostpreferably between 100 ppm and 200 ppm. “Residual initiator-free” isunderstood to mean that the amount of the initiators used for the syruppolymerization is below 0.001%. The amount of carbocyclic compound isapprox. 100 ppm for sheets with a thickness of 30 mm, 140 ppm for sheetswith a thickness of 100 mm and 250 ppm for sheets of 200 mm.

The amount of residual initiator is determined to be <5 ppm. The term“residual initiator-free” relates to the initiators used for thepolymerization. In the polymerization, a side reaction forms methylmethacrylate peroxide (MMA peroxide). This MMA peroxide decomposes athigh temperature (half-life>100° C.) and does not disrupt the subsequentpolymerization at from approx. 25° C. to 40° C.

EXAMPLES

Preparation of a Residual Initiator-Free Syrup by Feed Polymerization

760 l of MMA are required per batch. For polymerization, 0.08% of aperoxydicarbonate, preferablybis(4-tert-butylcyclohexyl)peroxydicarbonate, is added. After goodmixing, half of the solution is initially charged in the heating vessel.The second half remains in the feed vessel. The heating operation isstarted in automatic operation. The solution is heated to 73° C. Afterthe heating phase has ended, the temperature rises to 86° C. At thistemperature, the feed of the second half of methyl methacrylate from thefeed vessel begins automatically between 18 and 30 l/min. Thetemperature should not exceed 93° C. After the feed has ended, thecontents are discharged into the cooling vessel. Depending on thecooling conditions, the polymer fraction is 10-30%. The viscositymeasured in 6 mm Ford cups is 30-60 seconds. The viscosity is determinedanalogously to DIN 53211.

Production of a 100 mm Block

By a known process, the solution to be polymerized is poured between twoglass sheets which are distanced with a plastic cord. The solutionconsists of 60-90% residual initiator-free syrup, 10-40% MMA and alsothe customary additives: the initiator, thermal and UV stabilizer,γ-terpinene (140 ppm) and a release agent. The polymerization iseffected at 27° C. After about 12 days, the end polymerization iseffected in a heating oven. Without syrup, i.e. with MMA alone, thepolymerization time in a waterbath is about 30 days. The time in theheating oven remains unchanged.

1. A process for producing plastics moldings with a thickness of 30-200mm by thermal polymerization of a mixture of a residual initiator-freepolymethyl (meth)acrylate syrup and a mixture consisting of MMA and thecustomary additives and an olefinic carbocyclic compound.
 2. The processas claimed in claim 1, characterized in that the residual initiator-freepolymethyl (meth)acrylate syrup has the following properties: content ofinitiator peroxides used: below the detection limit (<5 ppm) averagemolecular weight M_(w) 240 000-350 000 composition: from 70 to 90% bymass of methyl methacrylate, from 10 to 30% by mass of PMMA viscosity:30-60 seconds (Ford cup).
 3. A process for preparing a residualinitiator-free polymethyl (meth)acrylate syrup, characterized in that amixture of: 100 parts of MMA and 0.05-0.1 part of peroxydicarbonate isincipiently polymerized up to conversion of 10-30%.
 4. Sheets of PMMAobtainable by a process of claim
 1. 5. The use of the sheets of PMMA asclaimed in claim 4 for producing aquaria.
 6. The process as claimed inclaim 1, characterized in that the carbocyclic compound is used inamounts of 50-300 ppm based on the amount of the polymerization batch.7. The process as claimed in claim 1, characterized in that thecarbocyclic compounds used are terpenes.
 8. The process as claimed inclaim 1, characterized in that the terpene used is γ-terpinene.